Referring to FIG. 1, screen assemblies 10 such as those used in wastewater treatment facilities, may provide panels that can be assembled together to form a windmill-like filter wheel 12. The filter wheel 12 may be partially submerged in wastewater so that some of the screen assemblies 10 extend into a flow of water 14 to provide a screening or filtering action. As the submerged screen assemblies 10 become dirty or clogged on account of their filtering nature, the filter wheel 12 is indexed 16 to move the dirty screen assemblies 10 out and clean screen assemblies 10 into the flow of water 14. The dirty screen assemblies 10 are cleaned for future use, for example, in place, after they have been rotated out of the water stream.
Each screen assembly 10 may, for example, include an outer frame 18 describing a planar isosceles trapezoid to approximate a sector of an annulus that forms the filter wheel 12. The frame 18 defines a filter area (typically exceeding 350 square inches) that is divided by a set of parallel stringers 20 into filter sections 21, each filter section holding a pleated mesh 22. The pleated meshes 22 in each filter section 21 are generally separate panels of woven mesh 23 that are joined together in the frame 18 by having their ends and edges in-molded into a thermoplastic material forming the frame 18 and stringers 20. The mesh 23 may, for example, provide a mesh size of approximately 11-30μ.
Generally, the width of the stringers 20, across which the adjacent pleated mesh 22 are joined, and as measured on an axis extending between the parallel sides of the frame 18 and perpendicular to the intended flow of water 14, is desirably minimized to maximize filter area and minimize flow resistance. Conversely the height of the stringers 20, along the direction of intended water flow, is sized to approximately equal the height of the pleats so as to fully capture and support the pleated ends of the pleated mesh 22.
In one prior art implementation, the pleated mesh 22 is of a woven mesh 23 and is pleated along parallel fold lines extending generally radially (with respect to the wheel 12 when the panel 10 is assembled into the wheel 10). The stringers 20, accordingly, extend generally perpendicular to fold lines 24 and parallel to tangents to the periphery of the wheel 12 when the panel 10 is assembled into the wheel 10.
Molded ridge bars 26 may follow the fold lines of the pleated mesh 22 adhered on both sides of the mesh 23 at ridges and valleys of the pleats to the pleated material to provide support for the pleats. During manufacture, the pleated mesh 22 is inserted into an injection mold, the latter providing cavities that form the frame 18, stringers 20, and ridge bars 26. Injection of thermoplastic into these cavities provides an in-molding of the pleated mesh 22 to assemble them into the panel 10.
A problem with this type of molding operation is that the mesh 23 may shift in the mold during the molding operation under the force of the flowing thermoplastic. This can be particularly a problem at the pleated ends of the pleated mesh 22 where they are inserted into the stringers 20. During use, screens that are poorly captured in the stringers can pull out 19 which, in turn, promote further ripping of the mesh 23 along the ridge bars 26.
Referring to FIG. 2a, this shifting may not always be easily detected particularly when the mesh 23 is partially embedded within the material of the stringer 20 over only a narrow edge region 28 less than half the width of the stringer 20. This is in contrast to the desired larger edge region 30 of the mesh 23 that should be embedded in the stringer 20 equal to substantially half the width of the stringer 20 as shown in FIG. 2b. This larger edge region 30 is necessary to provide good retention of the ends of the mesh 23.
Visual inspection may indicate no difference between the two assemblies of FIGS. 2a and 2b. The ability to ensure proper location of the woven mesh 23 during molding is particularly critical given the desirably narrow width of the stringers 20 for conservation of plastic.